A scientific analysis of a rare polar bear fossil indicates that the large, white-coated mammals evolved in the relatively recent past from common brown bears. The discovery suggests polar bears' ancestors migrated toward the North Pole in response to global warming thousands of years ago, and adapted quickly to their new Arctic habitat.

Scientists analyzing the rare fossil, found in Norway's northern Svalbard islands in 2004, conclude that polar bears appear to have evolved from brown bears 150,000 years ago - a mere blink of an eye on the earth's geological timeline.

The genetic evidence comes from an unusually well-preserved polar bear jawbone with a canine tooth still attached to it - a rare find because most polar bear carcasses are consumed by scavengers or sink to the sea bottom. The DNA mapped from the jawbone is the earliest mammalian genome ever sequenced.

Researchers have long suspected that the arctic-dwelling animals evolved from brown bears because of their present-day genetic similarities. But scientists could not confirm an evolutionary timeline. Estimates of the origins of polar bears have ranged all the way from 150,000 years ago to one million years ago. But a comparison of the DNA in the polar bear fossil and in a group of modern brown bears living on a group of islands off the Alaskan coast showed the two genomes more closely related than they are today.

The researchers were able to estimate that the polar bears' ancestors branched off from the brown bear population when they headed north and adapted to the Arctic habitat, becoming a separate species about 150,000 years ago.

Charlotte Lindqvist, an evolutionary biologist at the University of Buffalo in New York, led the research. Lindqvist says the fossil appears to be that of an adult male approximately the same size as modern polar bears.

"We found that it probably had a diet similar to polar bears today. And from the stratus [sic] [stratum or layers of rock or sediment] from where the fossil was found, we can see that it lived in an environment probably similar as today," she said. "So, this means that very rapidly polar bears probably adapted to a habitat very similar to what we see today," she added.

Lindqvist and colleagues theorize that one reason for the migration of brown bears to the Arctic may have been to escape the interglacial warming of the late Pleistocene period.

It's possible that Svalbard, where the fossil was found, served as a refuge for bears attempting to survive rising temperatures during this climate change, according to co-researcher Stephan Schuster of the University of Pennsylvania Center for Comparative Genomics.

"It is questionable how they would survive this, and I think the most logical scenario would be that they were following the cold weather," said Schuster. "And Svalbard would have been this kind of environment, where the polar bears could have gone and survived this warming period," he said.

Even today, notes the University of Buffalo's Charlotte Lindqvist, brown bears have been spotted wandering into areas considered to be polar bear habitats in the Canadian Arctic, suggesting that current warming trends may once again be impacting brown bear populations.

Lindqvist and her research team plan further genetic analysis of the polar bear fossil, which should yield clues about the migration routes taken by brown bears to the Arctic and the bears' rapid adaptation to the polar environment.

Additional studies could also tell researchers at what point during its evolution the polar bear developed its white coat, which Lindqvist says is actually colorless but looks white against the snowy landscape.

"By recovering more of the genome, we may be able to answer more questions about their evolution and their origin and how they adapted to their current habitats. And perhaps something about how they may be able to adapt to future changes," said Lindqvist.

An international team of researchers described the polar bear fossil in the Proceedings of the National Academy of Sciences.